Organic compounds and carbonyl compounds of the transition metals, in particular compounds containing a metal of the platinum group as the central atom, are being increasingly used as catalysts in industrial chemical processes. The recovery of the catalyst has a considerable influence on the economy of these processes. It should be recovered as completely and as simply as possible.
Therefore, there have been many attempts to develop techniques which satisfy these requirements. Two main routes have been pursued. The first restricts itself to recovering the catalyst metal and accepts the degradation of the catalytically active compound. Thermal cleavage, reduction, oxidation and precipitation steps dominate these processes. The aim of the other route is not to allow the catalytically active metal compound to be destroyed, but to recover it undamaged so that it can be recycled without any further treatment. The following deals in greater detail with this second variant for the recovery of organometallic compounds or metal carbonyls.
One possible route for separating organometallic coordination complexes from organic liquids consists in the use of selective separating membranes. Thus, DE-OS 19 12 380 describes a process in which a mixture of the complex with one or more organic components is brought into contact under pressure with one side of a cellulose membrane. The ratios of the molecular size and shape of the complex to molecular size and shape of the organic components are such that the material diffusing through the membrane has a reduced complex content.
DE-OS 19 53 641 describes a process for separating organometallic compounds from a solution of the compounds in an organic solvent by means of a membrane. This procedure is characterized in that a silicone rubber membrane is used. Furthermore, according to a process described in GB-PS 12 66 180, organometallic compounds are removed from their solutions in organic solvents by means of a polyamide membrane. Finally, DE-OS 24 14 306 describes the separation of organometallic compounds from organic solutions with a polyacrylonitrile membrane.
However, the aforementioned separating processes have the disadvantage that the membranes are not stable in the organic solvents, some of which attack the membranes; in particular, they swell when subjected to pressure and temperature loading and thus lose their beneficial properties. For this reason, no membrane process for such separations has been able to establish itself in industrial practice.
Therefore, the problem was to develop a process which permits the separation of organometallic compounds and/or metal carbonyls from organic media using membranes which not only exhibit the required separating properties under the selected operating conditions, but also are highly stable and thus permit simple and effective separation of the organometallic compounds or the metal carbonyls from organic media.